Re: [PATCH v3 6/7] iomap: remove old partial eof zeroing optimization

["Darrick J. Wong" <djwong@kernel.org>]

On Tue, Jul 15, 2025 at 12:20:14PM -0400, Brian Foster wrote:
> On Tue, Jul 15, 2025 at 07:37:33AM -0700, Darrick J. Wong wrote:
> > On Tue, Jul 15, 2025 at 08:36:54AM -0400, Brian Foster wrote:
> > > On Mon, Jul 14, 2025 at 10:34:17PM -0700, Darrick J. Wong wrote:
> > > > On Mon, Jul 14, 2025 at 04:41:21PM -0400, Brian Foster wrote:
> > > > > iomap_zero_range() optimizes the partial eof block zeroing use case
> > > > > by force zeroing if the mapping is dirty. This is to avoid frequent
> > > > > flushing on file extending workloads, which hurts performance.
> > > > > 
> > > > > Now that the folio batch mechanism provides a more generic solution
> > > > > and is used by the only real zero range user (XFS), this isolated
> > > > > optimization is no longer needed. Remove the unnecessary code and
> > > > > let callers use the folio batch or fall back to flushing by default.
> > > > > 
> > > > > Signed-off-by: Brian Foster <bfoster@redhat.com>
> > > > > Reviewed-by: Christoph Hellwig <hch@lst.de>
> > > > 
> > > > Heh, I was staring at this last Friday chasing fuse+iomap bugs in
> > > > fallocate zerorange and straining to remember what this does.
> > > > Is this chunk still needed if the ->iomap_begin implementation doesn't
> > > > (or forgets to) grab the folio batch for iomap?
> > > > 
> > > 
> > > No, the hunk removed by this patch is just an optimization. The fallback
> > > code here flushes the range if it's dirty and retries the lookup (i.e.
> > > picking up unwritten conversions that were pending via dirty pagecache).
> > > That flush logic caused a performance regression in a particular
> > > workload, so this was introduced to mitigate that regression by just
> > > doing the zeroing for the first block or so if the folio is dirty. [1]
> > > 
> > > The reason for removing it is more just for maintainability. XFS is
> > > really the only user here and it is changing over to the more generic
> > > batch mechanism, which effectively provides the same optimization, so
> > > this basically becomes dead/duplicate code. If an fs doesn't use the
> > > batch mechanism it will just fall back to the flush and retry approach,
> > > which can be slower but is functionally correct.
> > 
> > Oh ok thanks for the reminder.
> > Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>
> > 
> > > > My bug turned out to be a bug in my fuse+iomap design -- with the way
> > > > iomap_zero_range does things, you have to flush+unmap, punch the range
> > > > and zero the range.  If you punch and realloc the range and *then* try
> > > > to zero the range, the new unwritten extents cause iomap to miss dirty
> > > > pages that fuse should've unmapped.  Ooops.
> > > > 
> > > 
> > > I don't quite follow. How do you mean it misses dirty pages?
> > 
> > Oops, I misspoke, the folios were clean.  Let's say the pagecache is
> > sparsely populated with some folios for written space:
> > 
> > -------fffff-------fffffff
> > wwwwwwwwwwwwwwwwwwwwwwwwww
> > 
> > Now you tell it to go zero range the middle.  fuse's fallocate code
> > issues the upcall to userspace, whch changes some mappings:
> > 
> > -------fffff-------fffffff
> > wwwwwuuuuuuuuuuuwwwwwwwwww
> > 
> > Only after the upcall returns does the kernel try to do the pagecache
> > zeroing.  Unfortunately, the mapping changed to unwritten so
> > iomap_zero_range doesn't see the "fffff" and leaves its contents intact.
> > 
> 
> Ah, interesting. So presumably the fuse fs is not doing any cache
> managment, and this creates an unexpected inconsistency between
> pagecache and block state.
> 
> So what's the solution to this for fuse+iomap? Invalidate the cache
> range before or after the callback or something?

Port xfs_flush_unmap_range, I think.

--D

> Brian
> 
> > (Note: Non-iomap fuse defers everything to the fuse server so this isn't
> > a problem if the fuse server does all the zeroing itself.)
> > 
> > --D
> > 
> > > Brian
> > > 
> > > [1] Details described in the commit log of fde4c4c3ec1c ("iomap: elide
> > > flush from partial eof zero range").
> > > 
> > > > --D
> > > > 
> > > > > ---
> > > > >  fs/iomap/buffered-io.c | 24 ------------------------
> > > > >  1 file changed, 24 deletions(-)
> > > > > 
> > > > > diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
> > > > > index 194e3cc0857f..d2bbed692c06 100644
> > > > > --- a/fs/iomap/buffered-io.c
> > > > > +++ b/fs/iomap/buffered-io.c
> > > > > @@ -1484,33 +1484,9 @@ iomap_zero_range(struct inode *inode, loff_t pos, loff_t len, bool *did_zero,
> > > > >  		.private	= private,
> > > > >  	};
> > > > >  	struct address_space *mapping = inode->i_mapping;
> > > > > -	unsigned int blocksize = i_blocksize(inode);
> > > > > -	unsigned int off = pos & (blocksize - 1);
> > > > > -	loff_t plen = min_t(loff_t, len, blocksize - off);
> > > > >  	int ret;
> > > > >  	bool range_dirty;
> > > > >  
> > > > > -	/*
> > > > > -	 * Zero range can skip mappings that are zero on disk so long as
> > > > > -	 * pagecache is clean. If pagecache was dirty prior to zero range, the
> > > > > -	 * mapping converts on writeback completion and so must be zeroed.
> > > > > -	 *
> > > > > -	 * The simplest way to deal with this across a range is to flush
> > > > > -	 * pagecache and process the updated mappings. To avoid excessive
> > > > > -	 * flushing on partial eof zeroing, special case it to zero the
> > > > > -	 * unaligned start portion if already dirty in pagecache.
> > > > > -	 */
> > > > > -	if (!iter.fbatch && off &&
> > > > > -	    filemap_range_needs_writeback(mapping, pos, pos + plen - 1)) {
> > > > > -		iter.len = plen;
> > > > > -		while ((ret = iomap_iter(&iter, ops)) > 0)
> > > > > -			iter.status = iomap_zero_iter(&iter, did_zero);
> > > > > -
> > > > > -		iter.len = len - (iter.pos - pos);
> > > > > -		if (ret || !iter.len)
> > > > > -			return ret;
> > > > > -	}
> > > > > -
> > > > >  	/*
> > > > >  	 * To avoid an unconditional flush, check pagecache state and only flush
> > > > >  	 * if dirty and the fs returns a mapping that might convert on
> > > > > -- 
> > > > > 2.50.0
> > > > > 
> > > > > 
> > > > 
> > > 
> > > 
> > 
> 
>